Apparatus for the measurement of a concentration of a charged species in a sample

ABSTRACT

An apparatus ( 1 ) for the measurement of a concentration of a charged species in a sample ( 10 ) is disclosed. The sample ( 10 ) comprises a plurality of types of charged species and at least one insoluble component. The apparatus ( 1 ) comprises a first circuit with a voltage control device ( 54 ) connectable to two first electrodes ( 30, 30 ′) arranged along a channel ( 12 ) holding the sample ( 10 ) and a second circuit with a conductivity detection device ( 55 ) connectable to two second electrodes ( 5, 5 ′) arranged in the channel ( 12 ). The first circuit and the second circuit are dc and ac electrically isolated from each other.

CROSS-REFERENCE TO OTHER APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 13/202,022 filed Oct. 11, 2011, which is a U.S. National Stageapplication of International Patent Application No. PCT/EP09/51874 filedFeb. 17, 2009. Each of the foregoing applications are herby incorporatedherein by reference.

FIELD OF THE INVENTION

The invention relates to an apparatus and method for sensing of chargedspecies in biological, chemical, industrial or environmental samples. Inparticular, the invention relates to a method and an apparatus formeasuring charged species concentrations, in particular ionconcentrations, for example lithium ion concentrations, in samples, suchas blood.

BACKGROUND AND RELATED ART

Inorganic ions are an essential requirement for life and are found inlarge amounts in drinking water, blood and every cell of an organism aswell as in the environment. For example, the concentration of many ionsi.e. sodium, potassium, magnesium, and calcium inside and outside ofcells is essential for any living organism. Consequently, the ionconcentration in the blood and blood cells of animals and human beingsalso is of high importance for a large variety of body functions.

Normally lithium is a trace element present in blood plasma. Lithium isalso used as a drug to treat bipolar mood disorder. It is estimated thatworldwide over one million people take lithium on a daily basis. Adisadvantage in the use of lithium is the very low therapeutic index,i.e. the ratio between the toxic concentration and the therapeuticconcentration. Most patients respond well to a blood plasmaconcentration of 0.4-1.2 mmol/L lithium while toxic effects can occur ata lithium concentration of above 1.6 mmol/L. A prolonged high bloodlithium level can even result in permanent damage to the nervous systemand even death. Monitoring of the lithium concentration during treatmentis therefore essential, with regular checks every couple of months tokeep the lithium level at desired level.

To avoid extensive operator handling, ion-selective electrodes (ISEs)are routinely used for measurements of blood parameters in an automatedfashion. These ISEs are fast and offer a large dynamic range. However,their response is logarithmic and the required high selectivity forlithium can be a problem. Additionally, in case of lithium intoxicationa fast procedure for blood analysis is required. Currently, a venousblood sample must be withdrawn from the patient by specially trainedpersonnel and transported to the central laboratory and the blood cellsneed to be removed before the measurement is made. This procedure cantake up to 45 minutes. To minimize sample throughput time and enablemeasurements on location, miniaturized devices employing ion-sensitivefield-effect transistors are available to determine the concentration ofpotassium and sodium in whole blood even as a hand-held analyzer.However, such analyzers are not used for lithium determination, becauseof the high background concentration of other charged species, inparticular sodium ions, compared to the much smaller concentration oflithium ions.

The direct measurement of lithium in whole blood and the determinationof inorganic cations in blood plasma have been described anddemonstrated by E. Vrouwe et al. in Electrophoresis 2004, 25, 1660-1667and in Electrophoresis 2005, 26, 3032-3042. Using microchip capillaryelectrophoresis (CE) with defined sample loading and applying theprinciples of column coupling, a concentration of alkali metals wasdetermined in a drop of whole blood. The whole blood collected from afinger stick was transferred onto a microchip without extraction orremoval of components of the whole blood. The lithium concentration canbe determined in the blood plasma from a patient on lithium therapywithout sample pre-treatment. Using the microchip with conductivitydetection, a detection limit of 0.1 mmol/L has been obtained for lithiumin a 140-mmol/L sodium matrix.

Other prior art documents disclosing several types of the microchips forthe measurement of the concentration of ions in the blood sample areknown in the art. For example, US Patent Application US 2005-0150766(Manz) discloses a capillary electrophoresis microchip.

U.S. Pat. No. 5,882,496 (Northrup et al) discloses a method forfabrication and use of porous silicon structures to increase a surfacearea of one of electrophoresis devices.

U.S. Pat. No. 7,250,096 (Shoji et al, assigned to HitachiHigh-Technologies Corp) teaches a method and apparatus for measuring acurrent-carrying path during electrophoresis to detect the state of thecurrent-carrying path.

One of the issues in the prior art is a formation of gas bubbles in theelectrolyte at the electrodes (as noted in U.S. Pat. No. 7,250,096)and/or undesired redox (reduction-oxidation) reaction due toelectrolysis at electrodes in a microchannel of the apparatus. Thisoccurs because the charge transport through the apparatus is carried byelectrons in an electric path and ions in a chemical path. The charge isexchanged between electrodes and ions at the electrodes.

The electrolyte in the microchannel has a specific gas capacity. Themaximum amount of the specific gas capacity is termed the gas limit. Thegas bubbles are formed when the gas limit is reached locally within themicrochannel. The formation of gas bubbles directly influences themeasurements.

The ions and other uncharged molecules undergo changes due to redoxreactions and changing concentrations at the electrodes. The gas bubblesare formed due to the formation of non-charged molecules which exceedthe gas limit and form gas bubbles. These gas bubbles are confinedwithin the microchannel of the device and as a result can distort themeasurements.

The formation of the gas bubbles can be avoided as is explained in priorart if there is a single electrical circuit for capillaryelectrophoresis measurement or a single electrical circuit for incontact conductivity detection and voltage and or current is controlledadequately. However, if there are two electrical circuits for themeasurement method combined, then the electrical interference of bothelectrical circuits adds complications.

Prior art methods of resolving this problem for single electricalcircuits include the use of alternating current between the electrodes,by limiting of the electrical current, by controlling the type of redoxreaction and by reducing the voltage below the redox potential. Limitingthe current can for instance be realized by using a current source,small channel geometries and low concentrations of the electrolyte in achannel. It is also possible to use a low concentration of backgroundelectrolyte in a channel. Furthermore the design of the electrodes canplay a role. Electrodes with large surface area are less susceptible tothe formation of gas bubbles since the charge concentration changes arespread over a larger area.

SUMMARY OF THE INVENTION

The invention provides an apparatus for the measurement of aconcentration of a charged species in a sample. The sample comprises aplurality of types of charged species and at least one insolublecomponent. The apparatus comprises a first circuit with a voltagecontrol device connectable to at least two first electrodes arrangedalong a channel holding the sample and a second circuit with aconductivity detection device connectable to at least two secondelectrodes arranged in the channel, wherein the first circuit and thesecond circuit are electrically isolated from each other

The electrical isolation means that the two electrical circuits do notinterfere with each other and therefore the measurements are accurate.

DESCRIPTION OF THE DRAWINGS

The invention may be better understood with respect to the figures andthe detailed description of preferred embodiments, which is illustrativeonly and not limiting to the invention and wherein:

FIG. 1a shows main components of an apparatus according to one aspect ofthe invention.

FIGS. 1b and 1c show arrangement of electrodes about a microchannel.

FIGS. 2a and 2b shows further arrangements of the electrodes about thechannel.

FIGS. 3a and 3b illustrate possible current paths at one electrode or inbetween electrodes.

FIGS. 4a and 4b show the connection of the components of the apparatusto the electrodes about the microchannel.

FIGS. 5a to 5d show different arrangements of isolation components.

FIG. 6 shows an example of a measuring device.

FIG. 7 shows an example of an apparatus with an expansion chamber

FIG. 8 shows an example of an apparatus with an sample conductivitymeasurement included

In the figures same reference numerals describe the same or similarobjects.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described with respect to the figures andexamples. It will be noted that features from one aspect of theinvention may be combined with features from another aspect of theinvention.

FIGS. 1a to 1c show the components of a measurement system 1 accordingto one aspect of the invention.

The measurement system 1 comprises a measuring device 17 which measuresand processes electrical signals from a sensor 18. The sensor 18measures a concentration of charges species in a liquid sample 10 (shownin FIGS. 1b and 1c ) and is disclosed more fully in the co-pendinginternational patent application no WO 2008/061542, the teachings ofthis patent application are fully incorporated herein. The liquid sample10 is most commonly a blood sample.

The sensor 18 has a chip holder 15 and a sample device 9. The chipholder 15 is disclosed more fully in international patent applicationno. PCT/EP2007/004468, the teachings of which are fully incorporatedherein. The sample device 9 is shown in more detail in FIGS. 1b and 1cand will be explained in more detail in conjunction with these figures.

The measuring device 17 has a sample conductivity measurement device 53,a voltage control and current sense device 54 and a conductivitydetection and cell control device 55. The conductivity measurementdevice 53 is connected to sample conductivity electrodes 4 and 4′ on thesample device 9 through electrical paths. The voltage control andcurrent sense device 54 is connected to reservoir electrodes 30 and 30′and to wall electrodes 30″ and 30″′ on the sample device 9 throughelectrical paths 60 to 60″′ respectively. Similarly the conductivitydetection and cell control device 55 is connected to channel electrodes5 and 5′ on the sample device 9 through electrical paths 65 and 65′.

A main control 43 in the measuring device 17 includes a processor 44 forperforming calculations. The main board 43 is connected to theconductivity measurement device 53 through an electrical path 75, to thevoltage control and current sense device 54 through an electrical path76 and to the conductivity detection and cell control device 55 throughan electrical path 77.

The measuring device 17 has an LCD display and buttons which areconnected to an operating panel 69. The operating panel 69 is connectedto the main board 43 through an electrical path 72. The measuring device17 is supplied with power through a power supply 68 connected to thepower supply 79. A serial port 67 is connected to the main board 43through an electrical path 73 and to an outside connection 78.

The sample device 9 comprises a substrate (not shown) into which achannel 12 is formed, as shown in FIG. 1a and more clearly on FIGS. 1band 1c . The substrate may be made from glass or plastics material. Anyother material allowing for the fabrication of channels 12 may be used.In case of glass as the substrate material, the channel 12 is etchedinto the substrate 13 between a first reservoir 8, a second reservoir 8′and a third reservoir 8″. The side walls of the channel 12 may be coatedwith a polymer. The channel 12 may be of sub-centimeter dimensions; inparticular the channel 12 may be less than 1 cm in width and less than100 μm in depth. The first reservoir 8, the second reservoir 8′ and thethird reservoir 8″ may be considerably larger in size than the width ofthe channel 12 (e.g. 100 μm to 1 cm. This can be seen in FIGS. 1b and 1c. Further one or more of the reservoirs may be included in the channel12.

The channel 12 and the first reservoir 8, the second reservoir 8′ andthe third reservoir 8″ may be filled with an electrolyte 11 prior touse. Typically the volume of the reservoir is around 10 ul.

FIGS. 1b and 1c show a side view of the sample device 9. The sampledevice in one exemplary embodiment of the invention has a width of 30mm, a height of 4 mm and a thickness of 1.4 mm. The chip can be made ofglass.

It will be seen that the channel 12 as well as the first reservoir 8,the second reservoir 8′ and the third reservoir 8″ have a number ofelectrodes. The channel 12 in one exemplary embodiment of the inventionis less than 100 um in width, has a depth of less than 100 um and alength of less than 3 cm. It will be further noted that a part 19 isconnected between the top surface 3 of the sample device 9 and thechannel 12. The sample 10 is placed on the top surface 3 of the sampledevice 9. The sample 10 is in fluid communication with a part 19 of thechannel 12 through an opening 2 in the top surface 3. The opening 2 andthe part 19 may have the form of a circle but any form suitable forinserting liquid into the channel 12 may be used.

More than one opening 2 may be made in the top surface 3. This isuseful, for example, for allowing the sample 10 to enter into thechannel 12 at multiple entry points. This allows for multiplemeasurements to be made and averages to be taken. One further advantageof more than one opening 2 is to allow convective flow from one openingtowards another opening and thus providing an alternative transportmechanism through the opening 2 into the channel 12. One furtheradvantage of more than one opening 2 is to prevent evaporation inchannel 12 as is disclosed in international patent application no.PCT/EP2007/004468.

The channel 12 is provided with a number of electrodes which havegenerally rounded corners to avoid concentration of current. Thereservoir electrodes 30, and 30′ are provided in the first reservoir 8and the third reservoir 8″. The reservoir electrodes 30 and 30′ allow avoltage to be placed along the channel 12. The reservoir electrodes 30and 30′ are connected to the voltage control and current sense device 54through electrical paths as explained above. The reservoir electrodes 30and 30′, as well as the well electrodes as described below, aretypically made of Platinum and are flat and thin, typically below 2 mmwidth and 2 mm length but a height in the order of 100 nm.

The top surface 3 and the other reservoir 8′ are provided with the wellelectrodes 30″ and 30′ which allow the placement of a voltage across thechannel 12. This is useful for drawing charged ions from the sample 10through the opening 2 into the cavity 19 and then into the channel 12.The well electrodes 30″ and 30′″ are connected to the voltage controland current sense device 54 through the electrical paths as explainedabove. A typical voltage used is 1200 V and a current would be less than10 uA.

The channel 12 has two channel electrodes 5 and 5′ which are situatedsubstantially opposite to each other and measure the conductivity acrossthe channel 12. The conductivity electrodes 5 and 5′ are connected tothe conductivity detection and control device 55 through electricalpaths as explained above. The two channel electrodes 5 and 5′ are around100 um apart and are also made of platinum. Their width is less than 100um, for example 40 um, and the two channel electrodes 5 and 5′ havemildly rounded corners. The signal applied across the channel istypically AC and in between 100 Hz and 100 kHz with an top-top amplitudebetween 1 and 10V.

The two channel electrodes 5 and 5′ allows the use of an in contact iondetection (abbreviated ICCD) mechanism into the apparatus 1. The ICCDmechanism is a detection method in which the channel electrodes 5 and 5′have a direct electrochemical interface with the fluid in the channel12.

FIG. 1c shows two of the sample conductivity electrodes 4 and 4′ on thetop surface 3. The sample conductivity electrodes 4 and 4′ are coveredby the sample 10 and measure the conductivity of charges species in thesample 10 before, during and/or after the charged species are drawn intothe part 19 of the microchannel. The sample conductivity electrodes 4and 4′ are connected to the sample conductivity measurement device 53,as explained above. The sample conductivity electrodes 4 and 4′ have agenerally rounded form which reduces the current density at the tips ofthe sample conductivity electrodes 4 and 4′.

FIGS. 2a and 2b show the arrangement of the electrodes in the channel 12in more detail. For simplicity the first reservoir 8, the secondreservoir 8′ and the third reservoir 8″ are not shown in detail. Onlythe electrodes 30 to 30″′ are shown. In FIG. 2a the channel electrodes 5and 5′ are not placed inside of the channel 12 but are outside of thechannel walls 7 and 7′. In other words neither of the channel electrodes5 and 5′ are in direct contact with the fluid 11 in the channel 12. InFIG. 2b it will be seen that the channel electrodes 5 and 5′ penetratethrough the side walls 7 and 7′ and are in fluid (and direct electrical)contact with the fluid 11 in the channel 12. The aspect of the setupshown in FIG. 2a has the advantage that neither of the two channelelectrodes 5 and 5′ are in direct contact with the fluid 11. As a resultit is not possible for gas bubbles to form on the surface of the twochannel electrodes 5 and 5′.

In the aspect of the invention shown in FIG. 2b it is necessary toensure that the voltage and or the type of redox reaction and or theelectrical current is controlled at the two channel electrodes 5 and 5′in that aspect that the formation of gas stays below the gas limit. Inan alternative aspect of the invention, an alternating current can bepassed across the two channel electrodes 5 and 5′.

FIGS. 3a and 3b show the current paths on the various electrodes 30 and30′, the current paths on the electrodes 30″ and 30″′ are not shown forclarity but are also present. FIG. 3a shows one exemplary channelelectrode 5 (or 5′) within the channel 12 (i.e. the aspect of theinvention shown in FIG. 2b ). The current paths 40 a and 40 d arepresent along the channel 12 towards the reservoir electrodes 30 and30′. The current paths 40, 40 c and 40 b act across the channel 12.

FIG. 3b shows the current paths acting on the reservoir electrodes 30and 30′ as well as on the channel electrodes 5 and 5′. It will be notedthat the reservoir electrode 30 has a current path 51 a in the directionof the channel electrode 5 and a current path 51 c in the direction ofthe channel electrode 5′ as well as a current path 51 b in the directionof the other reservoir electrode 30′. Similarly the reservoir electrode30′ has a current path 51 d in the direction of the channel electrode 5and a current path 51 f in the direction of the channel electrode 5′ aswell as a current path 51 e in the direction of the other reservoirelectrode 30.

The channel electrode 5 has a current path 50 a in the direction of thereservoir electrode 30 and a current path 50 c in the direction of thereservoir electrode 30′ as well as a current path 50 b in the directionof the channel electrode 5′. The channel electrode 5′ has a current path50 d in the direction of the reservoir electrode 30 and a current path50 f in the direction of the reservoir electrode 30′ as well as acurrent path 50 e in the direction of the channel electrode 5.

FIGS. 3a and 3b illustrate one of the problems in combining capillaryelectrophoresis methods for separating the ions with in contactconductivity detection. Not only are the electrical potentials betweenthe channel electrodes 5 and 5′ and between the reservoir electrodes 30and 30′ relevant, but it is also necessary to consider “cross electrode”or “cross mechanism” current paths given by the reference numerals 50 a,50 c, 50 d, 50 f and 51 a, 51 c, 51 d, 51 f. There needs to be isolationbetween the circuit including the channel electrodes 5 and 5′ and thecircuit formed from the circuit including the reservoir electrodes 30and 30′. This issue is more acute in small scale apparatus, such as thatof the invention.

This can be done by electrically ensuring that there is no or reducedcommon dc or ac connection through the electronics. This is illustratedin FIGS. 4a and 4b which show in the top halves as an electricalisolator 80 near the circuit for voltage control 54 and in the bottomhalves as an electrical isolator 80′ near the circuit for theconductivity detection 55.

The reservoir electrodes 30 to 30″′ are connected by the electricalpaths 60 to 60″′ to the voltage control and current sense device 54. Thewell electrodes 5 and 5′ are connected to the conductivity detection andcell control device 55 through the electrical paths 65 and 65′.

In FIG. 4a the voltage control and current sense device 54 is connectedto the main board through an electrical isolator 80. In FIG. 4b theconductivity detection and cell control device 55 is connected to themain board 43 through an isolator 80′. The purpose of the electricalisolators 80 and 80′ is to isolate the various electrodes from eachother. The electrical isolators 80 and 80′ are shown in an exemplaryposition in FIGS. 4a and 4b , but it will be noted that they can beplaced in other positions. It will also be note that more than oneelectrical isolator 80 and 80′ can be included. In general it can bestated that an electrical isolator 82 with a low capacitance has to berealized between the voltage control circuit 54 and the conductivitydetection circuit 55 in combination with the electrodes 30 to 30″′ and 5to 5″.

The electrical isolators 80 and 80′ can have various configurations asshown in FIGS. 5a to 5d . In FIG. 5a the input of each of the twoelectrical paths 90 and 91 is isolated from the output of the twoelectrical paths 90′ and 91′ by a capacitor 95 and 95 a. Similarly inFIG. 5b the input of each of the two electrical paths 90 and 91 isisolated from the output of the two electrical paths 90′ and 91′ by aninductor 96. In FIG. 5c an inductor 97 has a central tap 92, 92′. InFIG. 5d a piezo element 98 is caused to isolate the input of theelectrical paths 90 and 91 from the output of the electrical paths 90′and 91′.

The isolators 80 and 81 have the effect of substantially reducing the dccurrent between the capillary electrophoresis circuit and the ICCDcircuit. One further problem that can arise is the presence of an accurrent between the capillary electrophoresis circuit and the ICCDcircuit. This can be reduced by using the electrical isolators 80 and80′ as well.

It will be noted that the reduction in the capacitance 82 isadvantageous for the apparatus in order to reduce dc and ac effects. Itis thought that a capacitance of less than 100 pF, for example, 20 pF isoptimal in order to be able to accurately measure the ions in thechannel 12.

FIG. 6 shows an example of the measuring device with a display 69. Thewidth 16″ is generally less than 50 cm and in one exemplary embodimentis 10 cm. The height 16″′ is less than 10 cm and in one exemplaryembodiment is 5 cm. The depth 16′ is less than 50 cm and in oneexemplary embodiment is 20 cm.

A further embodiment of the invention is shown in FIG. 7 in which anexpansion chamber 104 is connected through an opening 100′ andinterconnection 101 to the part 19 of the microchannel 19 at entry 100′.It will be noted that the interconnection 101 has a cross section whichis smaller than the cross section of the channel 12. It will be furthernoted that the interconnection is long and curves back and forwards inorder to use the least amount of space. This is useful in order toincrease the aerodynamic resistance of the interconnection 101 to theexpansion chamber 104 with respect to the channel 12 and the otherreservoirs.

The expansion chamber 104 is filled with a fluid 102, such as theelectrolyte 11. A gas bubble 103 is present inside of the fluid 102. Thegas bubble 103 is preferably made of a gas, such as an inert gas, e.g.helium, argon.

The expansion chamber 104 with the gas bubble 103 allows the fluid inthe microchannel 12 to expand and shrink due to temperature differenceswithout destroying the chip. Temperature differences of a few degreesbut also for instance 50 degrees can be dealed with in this manner. Thisis necessary to enable efficient operation of the apparatus. If, forexample, the fluid in the microchannel 12 expanded so much that thefluid leaked out of the chip, then on cooling the microchannel 12 wouldno longer be completely filled with the fluid which would lead to achange in measurements. It will be noted that the expansion chamber 104has a substantial volume in comparison with the volume of reservoirs 8to 8″ and channel 12 in that manner that the gas bubble 103 created iscapable of resisting the temperature differences and preventing leakage.

The gas bubble 103 is generated by evacuating air from the sample device9 and then adding the inert gas to the microchannel 12 that seeps intothe expansion chamber 104. The sample device 9 is then evacuated againand the fluid is placed at the sample device 9. The sample device 9 isthen brought to atmospheric pressure and the fluid enters the sampledevice. Due to the residual gas in the expansion chamber 104 a gasbubble 103. In another method, the gas bubble 103 can be formed byelectrolysis of water. This requires, of course, electrodes to bepresent in the expansion chamber 104. The amount of evacuation of theexpansion chamber 104 governs the formation of the gas bubble 102. Thehigher aerodynamic resistance of the interconnection 101 means, forinstance, that gas leaks out more slowly from the expansion chamber 104than from the channel 12. This means that it is possible tosubstantially evacuate the channel 12 but still have some gas left inthe expansion chamber 104. On filling of the sample device with thefluid, the remaining gas left in the expansion chamber 104 forms the airbubble.

It will be noted that the use of the expansion chamber 104 issubstantially greater than in the described sample device 9. Forexample, the expansion chamber 104 can be used in other microfluidicdevices to compensate for the expansion/shrinkage of fluid incorporatedinto the microchannels of the microfluidic devices.

The sample conductivity can also be measured with another sample system120. In FIG. 8a typical aspect is shown of this sample system 120. Inthis aspect of the invention a sample entry 112 is implemented connectedby a channel smoothening 113 and a sample channel 111 to a samplereservoir 110. The sample reservoir 110 is typically open to air. Forthis aspect of the invention there is typically no direct connection tothe channel system 12.

The sample system 120 is typically dry prior to use. The filling of thesample system 120 after applying a sample 10 on the surface top isachieved through the sample entry 112 and the channel smoothening 113.This prevents the formation of gas bubbles and to allow proper fillingof the sample channel 111 around the electrodes 4 and 4′. The filling isachieved by for instance hydrodynamic pressure made possible by anopening to air in the sample reservoir 110.

Care has to be taken for the correct filling of the sample system 120due to the combined usage with the filled channel system 12. The samplesystem 120 is during production filled with the electrolyte 11. Theelectrolyte 11 has to be removed from the sample system 120. Thisremoval is done through the open sample reservoir 110 that is used todry the electrolyte 11 to air. In this aspect care has to be taken thatno sedimentation is created in the sample system 120 during theevaporation of the electrolyte 11 because this will effect the latersample filling. An evaporation chamber 115 is implemented connected by aevaporation channel 116 to the sample system 120. The entry of theevaporation channel 116 is placed close to the sample entry 112. Theevaporation chamber 115 is typically a closed chamber. Due to theevaporation chamber 115 and evaporation channel 116 the evaporation ofthe electrolyte 11 will terminate in the evaporation chamber 115 andtherefore the sedimentation of species will take place inside theevaporation chamber instead of inside the sample channel system 120.

It will be noted that the use of the sampling channel system 120,evaporation channel 116 and evaporation chamber 115 is substantiallymore greatly application than use in the sample device 9 disclosed inFIGS. 1a-1c . For example, the sampling chamber 120 with the evaporationchannel 116 and the evaporation chamber 115 can be used in other microfluidic devices to determine for instance the sample conductivity andplasma conductivity. An example is the measurement of the haemoglobinlevel.

The invention has been described with respect to several embodiments. Itwill, however, be clear to those skilled in the art that the inventionis not limited thereto. Rather the cope of the invention is to beinterpreted in conjunction with the following claims.

The invention claimed is:
 1. Method of fabricating an apparatus,comprising the steps of interconnecting a microfluidic channel systemwith an expansion chamber, for fluidly connecting the expansion chamberwith the microfluidic channel system, partially evacuating themicrofluidic channel system and the expansion chamber, covering themicrofluidic channel system with a fluid, increasing the pressure at themicrofluidic channel system and the expansion chamber, filling at leastpartially the microfluidic channel system with the fluid, filling atleast partially the expansion chamber with the fluid, forming a gasbubble in the fluid present in the expansion chamber, closing thechannel system, thereby obtaining a closed microfluidic channel system.2. Method according to claim 1, comprising the step of adding an inertgas to the microfluidic channel, the inert gas seeping into theexpansion chamber.
 3. Method according to claim 1, comprising the stepof partially evacuating an inert gas from a sample device.
 4. Methodaccording to claim 1, wherein the gas is air.
 5. Method according toclaim 1, wherein the step of forming the gas bubble comprises a step ofwater electrolysis.
 6. An apparatus comprising: an open and unfilledmicrofluidic channel; at least two electrodes placed at the open andunfilled microfluidic channel; a sample opening; an open samplereservoir; a closed evaporation chamber.
 7. The apparatus according toclaim 6, wherein a channel smoothening is located between the sampleopening and the open and unfilled microfluidic channel.